1. Field of the Invention
The present invention relates to an image forming method for visualizing a digital image signal having numerous values, and, more particularly, it relates to an image forming method for forming an image, having a plural types of .gamma. (gamma) features.
2. Related Background Art
Conventionally, it is known to use a copying machine of digital type for such image forming method. In the well-known fundamental construction of such a digital copying machine, the density distribution of an original is read by a reader portion comprising a lens, mirrors, an original illuminating lamp and a CCD, image information of the original is converted to a digital image signal having numerous values and various treatment are effected regarding the digital image signal, and then, the treated digital image signal is visualized by a printer portion comprising the combination of a laser scanner device including a laser, a polygon mirror and a laser modulating device, and a well-known electrophotographic device. That is to say, the digital copying machine of this type is constituted by the reader portion, an image signal treating portion and the printer portion.
One of the features of such image forming apparatus is a .gamma.-feature. The .gamma.-feature indicates a relation between the read density value of the original image and an output image density value outputted in correspondence to the density value. In the conventional digital copying machine, it is known to freely determine the .gamma.-feature by a table converting treatment effected in the image signal treating portion (hereinafter, the treatment for freely determining the .gamma.-feature is referred to as ".gamma. converting treatment"). Among image forming apparatuses using such technique, there is a digital copying machine wherein an image signal treating portion has a plurality of .gamma. converting functions to obtain the optimum image output in correspondence to a kind of original to be copied (for example, character original, network dot image original, silver salt photographic image or the like) and the .gamma.-feature is selected in dependence upon the kind of original.
Generally, in case of the character original, by forming an image with using the non-linear .gamma.-feature having the sharp building-up as shown in FIG. 3, it is possible to obtain an easily readable output image having high contrast by reproducing character lines having low density value with high density value. On the other hand, in case of the network dot image original and the silver salt photographic original, by forming an image with using the linear .gamma.-feature as shown in FIG. 4, it is possible to obtain an output image having high gradient and good density value reproductivity.
Further, in the digital copying machine, it is also well known to effect the treatment for changing the sharpness of the image regarding the image signal at the image signal treating portion in order to eliminate the spatial quantization error of the CCD. The concrete means for changing the sharpness may be a method wherein, regarding the density value of each pixel in the image, new density value for each pixel is determined by adding a value obtained by multiplying the spatial secondary derivative value of the density distribution around the each pixel by a predetermined coefficient to the density value of each pixel (this method is referred to as "secondary derivative adding treatment" hereinafter). In the secondary derivative adding treatment, when the coefficient by which the secondary derivative value is multiplied is a large negative (minus) value, the sharpness is emphasized to clarify the image; whereas, when the coefficient is a large positive (plus) value, the sharpness is weakened to make the image out of focus. In the conventional digital copying machine, the secondary derivative adding treatment for emphasizing the sharpness in order to reproduce the character original having the high frequency of use as an easily readable sharp image was effected.
However, in the image forming apparatus having the above-mentioned image treating means, the following disadvantages occurred. That is to say, when the original image is formed gathering the periodical image pixels (such as the network dot image), the period of the network spots is interfered with the reading period (cycle) of the reader portion and/or the writing period of the printer portion, thereby creating the moire fringe pattern in the output image. Such moire fringe pattern is worsened as the periodicity of the reader portion and/or the writing periodicity of the printer portion are emphasized. Therefore, when the sharpness is emphasized, since the periodicity of the each pixel in the image is emphasized, the moire fringe pattern is worsened. Accordingly, in the conventional digital copying machine wherein the character image is mainly handled and which has the tendency for emphasizing the sharpness, when the network dot image original was copied, the strong moire fringe pattern was generated in the output image, and, thus, the good quality image could not be obtained.
Further, regarding the output image obtained by effecting the strong secondary derivative adding treatment with respect to the network dot image original, the density of the image could not be reproduced surely. That is to say, as mentioned above, the secondary derivative adding treatment is the treatment for changing the density value of each pixel. Particularly, in the vicinity of the small dot having the high density, since the secondary derivative value is great, the difference in the density value before treatment and the density value after treatment becomes great.
In FIG. 5, the image density distribution before treatment is shown by the solid line, and a value obtained by multiplying the secondary derivative value of the image density distribution by the predetermined minus coefficient is shown by the broken line. FIG. 6 shows the image density distribution (after treatment) obtained by effecting the secondary derivative adding treatment regarding the image density distribution before treatment. Since the network dot image is formed by gathering the great number of dots, it should be noted that the density value added to a unit area of the image becomes great.
In order to eliminate the drawbacks of the aforementioned conventional digital copying machine, there has been proposed a method wherein the density of the image pixels in an image forming means is switched between the case where an image is formed with the .gamma.-feature for the character original and the case where an image is formed with the .gamma.-feature for the network dot image original and the silver salt photographic original. For example, the image was formed with 400 dpi regarding the character original and with 133 dpi regarding the network image original and the silver salt photographic original. However, this conventional technique had a disadvantage that the resolving power of the output image was worsened in the network image original and the silver salt photographic original mode.